The present invention relates generally to water heaters and water tanks and more particularly to the sacrificial anode in a water tank.
Water heaters have a variety of applications. A common use is for the heating of water in a residential structure. A water heater has a water tank, typically manufactured of steel. The water tank is typically cylindrical to withstand pressure and for simplicity of manufacture. The tank is oriented with the axis of the cylinder vertical so that a firebox may be positioned beneath the water tank and an exhaust stack may extend along the cylindrical axis through the water tank. To decrease heating costs, the water tank is often insulated.
To protect a steel water heater tank from corrosion, the interior of the tank is coated with a glass lining formed by coating the tank interior with an enamel and heating the tank to form the glass lining. However, small voids or cracks may be present as manufactured or may form later in the glass coating, possibly exposing the metal tank walls to corrosion. To protect against corrosion a sacrificial metal anode rod attached to a threaded fitting is screwed into a threaded boss at the top of the tank.
Cathodic corrosion is an electro-chemical phenomenon in which a metal tends to return to its native oxide state. This corrosion can be slowed, and even stopped, by making the surface to be protected the cathode of an electrochemical cell. Thus, an anode must be supplied that has a more negative electrochemical potential than the potential of the steel surface to be protected. Anodes are typically made of alloys of zinc, magnesium, and aluminum to protect steel surfaces.
Because of the sacrificial nature of the anode, it is necessary to replace the anode periodically as it is depleted. However, access to the anode mounting is hampered when the fitting is buried in insulation which covers the top of the tank. Moreover, if threads on the anode and the tank are not isolated from the water in the tank, the threads could become corroded, making the removal of the anode difficult. What is needed is a sacrificial anode which mounts with good electrical contact to the tank yet which is readily and expeditiously replaceable.